Natural killer susceptibility of brain tumor cell lines inversely correlates with the degree of cell differentiation and not with the level of human histocompatibility antigen expression

Histocompatibility antigens and natural killer susceptibility

The central question of the nature of the structure(s) involved in the recognition of targets by natural killer (NK) cells remains unresolved. Although NK-mediated cytotoxicity is not MHC-restricted, it has been suggested that these cells could recognize the targets more effectively in the absence of MHC class I antigens. In this paper we review the contradictory results obtained when studying the NK susceptibility of cell lines which constitutively express different levels of MHC antigens, or which have been induced to express MHC antigens by gene transfection or gamma-interferon treatment. Taken together, the results indicate that MHC antigens play a differential role in NK lysis depending on the nature of the target cells used; MHC class I antigens play a role in the NK resistance of cells from a hematopoietic lineage, but this does not extend to cells from other origins. The data reviewed also support the hypothesis that MHC class I antigens induced NK resistance by interfering with target structures, and that multiple NK molecules are involved in NK-mediated lysis as part of a possible advanced recognition system.

Sera from patients with colon, breast and lung cancer induce resistance to lysis mediated by NK cytotoxic factors (NKCF)

Natural killer (NK) cells are involved in the antitumoral immunologic mechanism. These cells act through the release of cytotoxic molecules defined as NK cytotoxic factors (NKCF). Inhibitory factors of NK and NKCF mediated lysis have been described in in vitro assays. This study evaluates the induction of resistance to NKCF cytotoxicity by sera from 27 patients with colon, breast and lung cancer. Addition of these sera to the cytolytic assay where K562 cells and concentrated NKCF were used, induced resistance to NKCF mediated cytotoxicity in 21 cases (77%). The sera from the group with metastasis blocked NKCF lysis more markedly than the group with local tumours. However, no differences were observed when the groups with colon, breast and lung cancers were compared. This blocking effect was not found to be related to gamma interferon (IFN) levels. In a previous study, we described a tumour factor (NK-RIF) produced by human cell lines derived from metastatic adenocarcinomas. This factor blocked lysis of tumour target cells by NK cells. Consequently, it is proposed that the release of similar tumour factors with a capacity to induce resistance to NKCF may be involved in tumour growth and metastatic spreading in in vivo.

Analysis of the mechanisms involved in NK resistance induced by a new tumor factor NK-RIF

The mechanisms involved in susceptibility or resistance of neoplasic cells to lysis by NK cells are not well known. We have recently described a 12-kDa factor (NK-RIF), produced and released by different tumor cell lines, making K562 resistant to NK lysis without affecting the cytotoxic function of NK effector cells. In this paper we further study the mechanism involved in NK resistance of K562 mediated by NK-RIF and its biological implications. The results show that NK-RIF does not affect the binding capacity of target and effector cells nor the levels of HLA class I antigen expression on the target cells, as a proof that resistance to NK-mediated lysis is not always associated with a defect in target effector binding or with an increased MHC class I antigen expression. However NK-RIF-treated K562 loses its capacity to induce NK cell activation and the subsequent capacity to release NKCF and makes K562 resistant to lysis by NKCF. Therefore our results show that induction of resistance to NK cytotoxicity can be the result of the modulation of target structures responsible for inducing effector cell activation without affecting target/effector binding molecules. This indicates that the structures involved in adherence and activation of NK cells have a different nature and that molecules other than HLA participate in NK resistance.

MHC class I expression on human tumour cells and their susceptibility to NK lysis

Although natural killer (NK) activity is not restricted by the major histocompatibility complex (MHC), it has been suggested that the level of expression of MHC antigens by target cells may influence their lysis by NK cells. We have studied the NK susceptibility of 20 cell lines obtained from primitive and metastatic human tumours and the K562 cell line treated with gamma-interferon, phorbol ester TPA and tumour factor NK-RIF. When the levels of MHC class I antigen expression on the human tumour cell lines and their NK susceptibility were compared, no relationship between these two parameters was observed. Furthermore the treatment of K562 with either gamma-interferon, TPA or NK-RIF decreased its NK susceptibility independently of MHC class I expression. These results indicate that the MHC class I antigen is not the only factor directly involved in NK susceptibility and suggest that other membrane structures modulated by gamma-interferon, TPA or NK-RIF may also influence NK susceptibility.